Fan clutches operating on viscous fluid torque transfer principles conventionally consist of a driving clutch plate coaxially mounted on a drive shaft that is driven at engine speed or some selected proportion thereof. A driven clutch body having a central bearing is generally coaxially located on the same drive shaft as the clutch plate to rotate at some range of speeds, effecting slip-speeds between the clutch body and the clutch plate. Both the clutch plate and the clutch body display intermeshing lands and grooves that act to transfer forces as the drive surfaces of the clutch. The space between the grooves is filled with a viscous fluid such as silicone that transfers drive torque from the clutch plate to the clutch body. A pump plate typically separates the interior of the clutch body into two chambers including a working chamber containing the drive surfaces and a reservoir chamber. A combination of raised diverters formed into or fastened onto the pump plate, and holes in the plump plate itself, form a pumping mechanism that uses the relative motion of the face of the clutch plate to force fluid out of the working chamber containing the drive surfaces, and into the reservoir chamber. This action removes the viscous fluid from the space between the drive surfaces, thereby reducing the driving torque transfer effected in the clutch.
A temperature responsive actuator is generally located on the face of a cover that seals the clutch mechanism. When subjected to temperatures higher than a preselected calibration temperature of the clutch, the actuator opens a valve formed into the pump plate and permits the fluid contained in the reservoir chamber to flow back into the working chamber. Once in the working chamber, centrifugal force causes the fluid to flow into the space between the drive surfaces, thereby restoring drive torque. The clutch pump and valve are sized so that when the valve is open, it returns fluid to the working chamber faster than the pump can remove it to the reservoir chamber, so that substantially all of the fluid carried by the clutch remains in the working chamber when the clutch is engaged.
Terminal disengage speed refers to the clutch output speed effected by residual fluid after fluid pump-out from the working chamber. With a viscous fluid clutch, conventional knowledge requires a relatively high terminal disengage speed of approximately 1.2-1.4 times engine speed, at least partially based on the premise that fan speed should never be lower than engine speed at idle. Generally, a disengage speed is required that is sufficient to supply cooling air flow to meet desired engine cooling temperatures and air conditioning compressor pressures, without completely engaging the clutch under all typical operating conditions. The fan drive is used to maintain air flow, pulling air through the heat exchangers under all conditions, even when the clutch is disengaged.
To minimize fan drive power consumption, disengaged fan speed is preferably minimized. Some cooling systems, such as those that use a viscous clutch driven fan in series with an electrically driven fan that supplies all but the most severe cooling requirements, would permit reducing the disengage speed. However, with a conventional viscous fluid fan clutch, an over-reduction in disengage speed results in inadequate filling for re-engagement. This is because the typical viscous fluid clutch relies on rotation of the output element, including the reservoir chamber, to provide impetus for moving the fluid to fill the lands and grooves during re-engagement. Accordingly, a required, relatively high minimum disengage speed stands as a barrier to reducing disengage speeds in a conventional viscous fluid clutch. Typically, fluid is constantly being pumped out from between the plates, from the working chamber, through pump holes, and into the reservoir chamber. This continuous fluid expulsion serves as another barrier to reducing disengage speed. If the pumping action were efficient enough to reach a very low disengage speed, it would remove fluid from the working chamber faster than it can be added to engage the clutch.
To set the disengage speed, a minimum fluid quantity that cannot be pumped out is generally maintained in the working chamber. To engage the clutch, additional fluid quantities are introduced to the working chamber by opening a port of the valve that is located radially inside the pump holes. Since centrifugal force tends to prevent the fluid from moving radially inward, other forces must be created to move the fluid to a position from which, it can move through the port and into the working chamber to engage the clutch. To enable the refilling function to engage the clutch forces must be applied to the fluid, in addition to the centrifugal force acting on the fluid as a result of rotation of the clutch. Accordingly, the reservoir chamber typically subjects the fluid to centripetal type forces to move the fluid radially inward against the centrifugal force. With the relative velocity (slip-speed), between the input and output elements determining the pump-out rate, and the rotational speed of the output element (which carries the reservoir chamber), determining the feed-in rate, too slow of a disengage speed can result in fluid being pumped out of the working chamber faster than it can be fed in. In other words, a conventional viscous fluid clutch will not operate if the disengage speed is too slow. With these conditions being present, the challenge exists to provide a viscous fluid clutch that operates with a low disengage speed that is steady state regardless of torque loading variances, and that operates to re-engage under very high disengage slip-speed ratio conditions.